The invention relates to a device for actuating a gas exchange valve having an electromagnetic actuator, which is floatingly supported in a cylinder head and abuts a play compensation element.
Electromagnetic actuators for actuating gas exchange valves usually have two operating magnets, specifically an opening magnet and a closing magnet, between the pole faces of which an armature is arranged such that it can be displaced co-axially in relation to a valve axis. The armature acts directly, or via an armature tappet, on a valve stem of the gas exchange valve. In the case of actuators operating in accordance with the mass-oscillator principle, a pre-stressed spring mechanism acts on the armature. Usually two pre-stressed compression springs, to be precise a top and a bottom valve spring, are used as the spring mechanism. The top valve spring acts in the opening direction, and the bottom valve spring acts in the closing direction of the gas exchange valve. When the magnets are not excited, the armature is retained by the valve springs in a position of equilibrium between the magnets.
When the actuator is initially activated, either the closing or the opening magnet is briefly over-excited, or the magnets are excited at the resonant frequency of the armature by an oscillation excitation routine, in order to be moved out of the equilibrium position. In the closed position of the gas exchange valve, the armature bears against the pole face of the energized closing magnet and is retained by the same. The closing magnet compresses further the valve spring, which acts in the opening direction. In order to open the gas exchange valve, the closing magnet is de-energized and the opening magnet is energized. The valve spring which acts in the opening direction accelerates the armature beyond the position of equilibrium, with the result that said armature is attracted by the opening magnet. The armature strikes the pole face of the opening magnet and is securely held by the same. In order to close the gas exchange valve again, the opening magnet is de-energized and the closing magnet is energized. The valve spring, which acts in the closing direction, accelerates the armature beyond the position of equilibrium toward the closing magnet. The armature is attracted by the closing magnet, strikes against the pole face of the closing magnet and is held by the same.
Variables, which have not been taken into account from the very beginning or which vary over time, such as production tolerances of individual components, rates of thermal expansion of different materials, differing levels of spring rigidity between the top and bottom valve springs, and settling of the springs as a result of the valve springs aging, etc., may result in a situation where the position of equilibrium determined by the valve springs does not correspond to an energetic center position between the pole faces or does not have a specific position. Furthermore, such variables may bring about situations where the armature no longer comes to bear fully on the pole faces of the magnets, where there is play between the armature stem and the valve stem and/or the gas exchange valve no longer closes completely.
An earlier application, DE 19 647 305.5, illustrates a play-compensation element in which an actuator is floatingly mounted in a cylinder head. The actuator opens and closes a gas exchange valve via an armature and two electromagnets, which are arranged on either side in the direction of movement of the armature. The spring mechanism is arranged between the actuator and the valve disc of the gas exchange valve. The top- that is the opening spring is supported on the actuator and the bottom- that is the closing spring is supported on the cylinder head. On the side remote from the gas exchange valve, a play-compensation element, which compensates both positive and negative valve play, is located between a cover plate and the actuator.
The play-compensation element has a first hydraulic element with a play-compensation piston in a cylinder. The play-compensation piston is located between a first pressure space, which is remote from the gas exchange valve and is controlled as a function of the internal combustion engine operating conditions, and a second pressure space, which is disposed adjacent the gas exchange valve. Located in the piston is a non-return valve, which is retained in the closed position by a closing spring. The non-return valve opens in the direction of the second pressure space when there is excess pressure in the first pressure space. The closing spring is configured such that the non-return valve does not open if there is no play, and thus interrupts the connection between the two pressure spaces.
Between the play-compensation piston and the cylinder there is a defined amount of play providing for a throttled path through which pressure medium can escape from the second pressure space. The play-compensation element is supported on the top cover plate, which is fixedly connected to the cylinder head. The play-compensation element may transmit either just compression forces or, in another embodiment, during the closing operation, compressive and tensile forces.
If the gas exchange valve does not close completely because the actuator is displaced too far in the direction of the gas exchange valve, i.e. there is negative play, the pressure in the second pressure space increases because a gas-exchange-valve valve spring acts in the closing direction. The pressure increase means that the pressure medium can escape from the second pressure space via the throttle path until the gas exchange valve closes again completely.
If the gas exchange valve closes correctly, but there is play between the armature tappet and the gas exchange valve, the valve spring of the gas exchange valve no longer acts on the second pressure space. The pressure in the second pressure space thus drops below that of the first pressure space, with the result that the non-return valve opens against the closing spring. The pressure medium flows from the first pressure space into the second pressure space until the play has been compensated for. This operation may last a number of working cycles of the valve. Since the position of the actuator changes during play compensation, the position of equilibrium of the valve springs thus also changes, with the result that it no longer corresponds to the energetic center position. This, however, changes the oscillating behavior of the spring mechanism, the energy requirement for the magnets and the opening and closing operations of the gas exchange valve.
It is the object of the invention is to provide a mechanism for actuating gas exchange valves having a play-compensation element, in which the position of equilibrium of the spring mechanism is independent of a displacement of the actuator.